1. Field of the Invention
The present invention relates generally to a gas turbine combustion system and a combustor ignition method therefor and more specifically to a gas turbine combustion system including a liquid fuel purge device and an ignition plug unit, a combustion ignition method therefor and a diffusion combustion type gas turbine combustion system.
2. Description of the Prior Art
As a prior art, there is used a combustor made such that a pilot nozzle is provided in a central portion of an inner cylinder of which end portion is open toward a gas turbine combustion chamber, a plurality of main nozzles are provided along a circumference surrounding the pilot nozzle and fuel supply rate to the plurality of main nozzles is regulated corresponding to change in the load of gas turbine so that the combustor is operated corresponding to size of load. It is to be noted that the term "fuel nozzle" as used herein is defined to include the main nozzles and the pilot nozzle.
For example, in the combustor of gas turbine used for power generation, fuel supply rate to the main nozzles is reduced or all the fuel nozzles including the pilot nozzle are extinguished to be stopped in the night time or the like time when the power demand is small. Also, in the day time or the like time when the power demand is large, diffusion flames are first generated by the pilot nozzle and then the main nozzles which had been extinguished and stopped are restarted to form active premixed flames. For this purpose, the main nozzles are regulated of the fuel supply rate so as to correspond to the size of load and are ignited for operation by the diffusion flames held in a flame holding cone provided in front of the pilot nozzle, so that the combustor is operated corresponding to the load.
Further, there is also used such one of the mentioned combustor as having same one fuel supply system constructed such that a liquid fuel and a gas fuel, both being usable therein, are supplied into the fuel nozzles according to the time by changing over the fuel, for example, from liquid fuel to gas fuel, during the operation.
FIG. 5 is a schematic view showing a combustor and a fuel system therefor in the prior art used for operation of a gas turbine. In this figure, a combustor 01 comprises; an inner cylinder 02 provided to a side wall of a combustion chamber 013 and having its front portion opening toward the combustion chamber 013; a fuel nozzle 05 consisting of a pilot nozzle 04 provided in a central portion within the inner cylinder 02 and a plurality of main nozzles 03 provided on a coaxial circumference surrounding the pilot nozzle 04; and a flame holding cone 012 provided in front of the pilot nozzle 04 for holding combustion of diffusion flames generated by the pilot nozzle 04.
Also, a liquid fuel system 08 comprises a main liquid fuel system 07 for supplying liquid fuel to the plurality of main nozzles 03 and a pilot liquid fuel system 06 for supplying the liquid fuel to the pilot nozzle 04.
Further, if the combustor 01 is made in a dual fuel type, that is, if the fuel nozzle 05 is so made that either liquid fuel or gas fuel is usable, the liquid fuel system 08 comprising the pilot liquid fuel system 06 and the main liquid fuel system 07 can supply not only the liquid fuel but also the gas fuel to the fuel nozzle 05 when the gas fuel is to be burned by the fuel nozzle 05. Also, in each of the liquid fuel system 08, that is, in each of the pilot liquid fuel system 06 and the main liquid fuel system 07, there is interposed a stop valve 09 for intercepting fuel supply to the fuel nozzle 05. It is to be noted that the term "fuel system" as used herein is defined to include the liquid fuel system and the fuel nozzle.
To each of the liquid fuel system 08 between the stop valve 09 and the fuel nozzle 05, there is connected an air purge system 010 having a stop valve 011 interposed therein, so that a fuel purge device is constituted thereby.
In this fuel purge device comprising the air purge system 010 and the stop valve 011, in case the combustor 01 is a dual fuel type combustor 01 for example, when gas turbine is to be stopped after liquid fuel has been used therefor, or when liquid fuel is to be changed to gas fuel while operation is being done using the liquid fuel, the stop valve 011 is opened and compressed air is thereupon supplied from the air purge system 010 into each of the liquid fuel system 08 between the stop valve 09 and the fuel nozzle 05 so that the liquid fuel remaining in said portion of the liquid fuel system 08 and in the fuel nozzle 05 is air-purged and is discharged into the combustion chamber 013.
However, in the prior art fuel purge device so supplying the compressed air to effect the air purge of the fuel remaining in the fuel system including the liquid fuel system 08 and the fuel nozzle 05, when the compressed air is injected into the fuel system, the liquid fuel remaining therein is discharged from the fuel nozzle 05 into the combustion chamber 013 suddenly by the effect of the compressed air so that there is a possibility of causing a sudden change of load.
Also, because of the purging using air which is lighter than the liquid fuel, there may be such a case where the remaining liquid fuel is not completely discharged but remains in the fuel system so that coking of the liquid fuel occurs due to high temperature of the surrounding area, which results in a problem of high possibility of causing a blockage of fuel passages in the fuel system including the liquid fuel system 08 and the fuel nozzle 05.
In FIG. 6, there is provided an ignition plug unit in the downstream area of said fuel nozzle 05. In this figure, an ignition plug 6 has a piston 8 provided to its back side portion and is constructed to be protrusile and retractile into and from within a combustor wall 1 by effect of air pressure acting on the piston 8 and elastic force of an elastic spring 7 resisting thereagainst so that it is protruded into within the combustor wall 1 only when the ignition is to be done, as shown by position B in FIG. 6.
On the other hand, a combustor in the upstream area of the position B within the combustor wall 1 is constructed such that fuel is supplied thereinto from a pilot nozzle 4 provided on a central axis of the combustor and from main nozzles 2 provided around the pilot nozzle 4 and air as combustion supporting gas is supplied through a pilot swirler 5 and main swirlers 3 provided around said respective nozzles so that the fuel and the air may be mixed and atomized.
In the combustor and the ignition plug unit so constructed, pressurized air from an air source 10 passes through a three way valve 9 and pushes the piston 8 provided to the back side of the ignition plug 6 against the elastic spring 7, thereby the ignition plug 6 is protruded through the combustor wall 1 to the position B from position A. On the other hand, the fuel and the air from the pilot nozzle 4 and the pilot swirler 5, respectively, are pilot-atomized and ignited by ignition spark from tip end of the ignition plug 6. Upon the ignition being so done, the air which had been supplied to the piston 8 via the three way valve 9 is drawn by the three way valve 9 being changed over and the ignition plug 6 is retracted quickly to the original position A from within the combustor wall 1 by the elastic force of the spring 7. Such protrusion and retraction of the ignition plug 6 are done instantaneously and the ignition spark is given only when the ignition plug 6 is in the position B.
If the combustor is of a dual fuel type in which both of gas and oil fuels are usable, there is a case where the position of the ignition plug tip which is optimal for the ignition is different between the gas fuel and the oil fuel. In this case, if the prior art ignition plug shown in FIG. 6 is used, there arises a problem where either one of these fuels may not be ignited.
FIG. 7 is a diagrammatic view showing an entire system of a prior art diffusion combustion type gas turbine which comprises a combustor having therein said fuel purge device, ignition plug, etc. sequentially from the upstream side. Numeral 21 designates a compressor, which is connected to a turbine 23 for mechanical drive via a rotational shaft (not shown) and has its upstream end communicating with an intake air passage 24. Numeral 22 designates a combustor, which has its upstream end communicating with an outlet portion of a downstream end of the compressor 21 and with a fuel passage 26 and also has its downstream end communicating with the turbine 23.
The turbine 23 communicates on its upstream side with the combustor 22, as mentioned above, and on its downstream side with an atmospheric discharge device of a stack and the like (not shown) or, according to a type of system, with a heat recovery device of a waste heat recovery boiler and the like (not shown), via an exhaust passage 25.
In take air from the intake air passage 24 is compressed by the compressor 21 to be mixed with fuel from the fuel passage 26 for combustion and resultant combustion gas is expanded at the turbine 23 for work at a generator and the like (not shown).
In the combustor of what is called a diffusion combustion type gas turbine shown in FIG. 7 into which fuel and air are supplied separately for combustion, there is an advantage that the combustor can be made less expensively as compared with a low NOx combustor of a premixing type which is generally compared with said diffusion combustion type and moreover the combustion is easily stabilized.
However, in the diffusion combustion type on one side, there occurs an area where the flame temperature becomes high due to diffusion of combustion and problem arises that NOx concentration becomes high.